The present invention relates to a method of forming an insulating film on, for example, the interconnection layer in an integrated circuit, or the like. The surface of the interconnection portions of the uppermost layer of multi-interconnection layers are oxidized to form a film of a metallic oxide, so as to enhance the moisture resistance of the interconnection layers and to enable the manufacture of a protective film or insulating film for the interconnection layers, so as to prevent deterioration of the moisture-resisting protective film which covers the uppermost layer of the interconnection portions.